1. Field of the Invention
The present invention relates to an image correction technique and, more particularly, to a technique of correcting image degradation caused by aberrations in an optical system used to capture an image.
2. Description of the Related Art
An image captured by an image capturing apparatus such as a digital camera is influenced by the chromatic aberration due to magnification and distortion aberration due to an optical system such as an imaging lens. Focusing on the pixels of the captured image reveals that an image that should be formed on a pixel of interest, in an aberration-free system, is moved, by the aberrations, and formed on the line that connects the pixel of interest and the center of the image.
Some recent image capturing apparatuses correct the influence of aberrations by image processing because the use of a multi-pixel image sensor or an optical system with a wide angle or a high zoom ratio makes image quality degradation, caused by the aberrations, apparent. As is known, the correction amount to be used to correct the information defining the pixel, which is moved due to aberration of the pixel, in an aberration-free system, is obtained based on the image height representing the distance from the image center and characteristics of the optical system.
Image movement due to the aberrations is not necessarily done for each pixel of the image sensor. The image is sometimes moved to a point (missing pixel) between the pixels of the image sensor. Hence, an interpolation operation is needed to correct for the influence of the aberrations. Japanese Patent Laid-Open Nos. 2001-186533 and 2005-057605 disclose techniques to derive the pixel value of a pixel of interest after correction by multiplying a plurality of pixels by a correction function so as to correct chromatic aberration due to magnification or distortion aberration in a captured image.
However, the above-described prior art make no mention of the correction limit based on the capacity of the storage area during correction processing, although a processing method of correcting for the influence of aberrations is disclosed. More specifically, in an image capturing apparatus including a multi-pixel image sensor as described above, a captured image is also growing in capacity. For this reason, to apply image processing by, for example, correcting for the influence of aberrations to the obtained image, it is necessary to divide the image into a number of regions, sequentially read out each region to the storage area, and then perform correction processing. In order to shorten the time required from image capturing to recording as in a video recording mode or the like, repetitive region readout needs to be avoided as much as possible. Image processing is required to be completed by the readout of one cycle per region. That is, since the aberration correction amount is limited by the capacity of the storage area, the correction needs to be applied in consideration of the correction limit.